Hmm...
This is not as simple as you think, nor as simple as inserting the IMG tag into your description.
Basically, if you want to use an existing Auctiva Template but have the text of your "Description" area be on top of an image, the only way I know of that will get you that look is to build the entire thing into a TABLE.
If you just use a standard IMG tag then your image will be in the description area along with your text, but your text will not be on top of the image...
So, to achieve a look where the text is on top of the image, and the image is the background, you'll basically have to build the entire description area in your own table.
Also note that there are some constraints that have to be taken into consideration as to the size of the image within the size of the table. If these measurements are not the same then the background image will TILE in most browsers (and may tile anyway in Netscape).
A quick Google Search on Background images in Tables will yield you some good information.
Ask away if you need more help...
Mike